The invention relates to optical sensor systems which precisely determine the correct angular orientation and lateral position of a component for precise placement of a component on a work surface by a component placement machine. More specifically, the invention relates to methods and apparatus for more efficiently aligning and placing components using optical component sensor systems.
Circuit boards that carry integrated electronic circuits as well as discrete electronic components are well known. To properly place an integrated circuit or other component on a circuit board, the leads of the component must be aligned to corresponding pads on the circuit board within a specified tolerance. The pattern of the pads on the circuit board is determined by the function of the circuit board and is designed on the circuit board prior to assembly.
The separation between centers of any pair of adjacent leads on electronic components is referred to as the pitch. Currently, a commonly manufactured lead separation is 0.025 inches (25 mil) pitch, meaning that the center of the leads are spaced at 25 thousandths of an inch intervals. Advances in component manufacturing technology, however, have produced integrated circuits having 15 and 10 mil pitches and tape automated bonding (TAB) components have been created having several hundred leads spaced with a 4 mil pitch. The bottom ends of the leads form a seating plane that will meet the plane formed by the pads on the circuit board when the component is placed in position.
The dimensions of components placed on circuit boards normally vary between 0.02 inch and 2.0 inches, although larger components may need to be accommodated. For quality manufacturing, component leads must be placed with at least 80% overlap of the lead onto the corresponding pad of the circuit board. For example, a device having a 20 mil pitch generally has 10 mil wide leads. With an 80% overlap, at least 8 mils of the lead width must be on the pad with no more than 2 mils of the lead width off the pad. In general, sensing systems used to align parts for placement must have five to ten times better resolution than the accuracy required. Therefore, 0.2 to 0.4 mil image resolution is required to achieve the maximum placement error of 2 mils specified for quality manufacturing methods for a component with 20 mil pitch. Correspondingly smaller image resolution is required for components with smaller pitch.
To perform this delicate task, precision surface mount component placement machines have been developed. While the particular design of the component placement machine is not relevant, all component placement machines generally pick up a component at one location, properly orient the component and place the component in its proper location on the circuit board. The components are not precisely aligned in the component bins where they are picked up. Therefore, components may be out of position by as much as plus or minus 50 mils and plus or minus 5 degrees angular orientation. To obtain proper placement, the orientation and lateral position of the components from the bins must be determined and then corrected prior to placement.
In a surface mount component placement machine, a component placement head picks up the component from a component bin utilizing a vacuum quill. The vacuum gently picks up the component to be placed and transports it between the component bins and the circuit board. A transport arm moves the placement head with the vacuum quill and the component from the bin to a circuit board located on a work table. Sometime during transport, the angular orientation of the component and the offset of the component from the center of the quill must be determined. The vacuum quill is then precisely lowered to fit the component on the circuit board. In current component placement machines, the transport arm and quill move at approximately one meter per second.
Assuming that the leads of the component have not been damaged, the position of the leads are known from the position and orientation of the body of the component. Therefore, if the lateral position and orientation of the body of the component are determined, the component can be properly placed. Alternatively, systems have been developed that can perform measurements on the leads of the component to adjust for irregularities in the leads during placement and to discover and reject damaged components prior to placement.
Mechanical systems have been commonly used to obtain correct angular orientation and lateral positioning of a component on the end of a quill. The mechanical contacting of the component can cause damage to the components. It is also extremely difficult to achieve the very high degree of accuracy both as to angular orientation and lateral position that is required by the design rules in use in today""s technology where lead spacing and widths are often only 10-25 mils wide. Therefore, non-contact, higher accuracy methods are desirable.
Conventional vision systems used in conjunction with component placement machines use solid state television cameras having a resolution of 512xc3x97512 picture elements or pixels. When viewing a two inch component, a corresponding two inch field of view with 512 elements produces a basic resolution of 4 mils or 4 thousandths of an inch. This is not sufficient resolution and, in fact, as pointed out above, it is necessary to achieve a resolution which is at least an order of magnitude greater. One solution is to use several cameras, but the use of several cameras is expensive.
Light based systems utilizing one or more focused light sources have been proposed which align a component by making a measurement of a shadow cast by the body of the component. U.S. Pat. No. 4,615,093, entitled Method and an Apparatus for the Positioning of Components With Reference to a Workpiece, describes several possible embodiments of focused light based component alignment systems. One of these embodiments, shown in FIG. 8 of the ""093 Patent, uses a row of laser diodes with sharply focused beams and a detector array with a detecting diode corresponding to each laser diode. The component is rotated until the number of diodes detecting laser light is maximized. This aligns the faces of the body of the component parallel to the sides of the frame around the measurement space.
U.K. Patent Number 2,183,820, entitled Electronic Component Placement, describes an alternative optical system for aligning a component based on the shadow that it casts. The system in the ""820 Patent uses two perpendicular light sources each with a corresponding array detector. When the machine is in the process of picking up the component, the machine waits until a shadow from the component is detected. If a component is not detected by the system, the system will try another attempt to pick up the component with up to three attempts made. To correctly orient the component, the component is rotated until the shadow cast on the first detector begins to lengthen. Rotation is then reversed until the shadow begins to lengthen again. Rotation is stopped when the component has its narrowest dimension oriented in a specific direction.
U.S. Pat. No. 5,278,634, assigned to one of the assignees of the present invention, entitled A High Precision Component Alignment Sensor System, incorporated herein by reference, discloses a non-contact, laser-based alignment sensor located on a placement head. The sensor is utilized to generate the correct angular orientation of the component for placement. The sensor also determines any offset in the X-Y plane of the center of the component with respect to the vacuum quill, which carries the component to the circuit board, to allow lateral alignment of the component. The high speed laser-based system disclosed in the ""634 Patent uses a stripe of laser light which is directed horizontally at the component whose alignment is being sensed.
The shadow cast by the component is detected by a linear array detector whose output is analyzed to detect the leading edge and the trailing edge of the shadow. This shadow edge information is analyzed as the component is rotated in the light beam to calculate the proper angular orientation and lateral alignment of the component. The procedure described in the ""634 Patent involves the rotation of the component until the shadow begins to increase. The shadow cast on the detector is smallest when the component is aligned along the light path. For determining the orientation of fine pitch components with their correspondingly smaller error tolerances, the component is then rotated at a slower speed in the opposite direction to obtain the proper alignment.
Copending U.S. patent application Ser. No. 08/289,279 (hereinafter the ""279 Application), filed Aug. 11, 1994, assigned to one of the assignees of the present invention, entitled High Precision Component Alignment Systems, incorporated herein by reference, discloses the use of several different optical systems with collimating lenses, condenser lenses, cylinder lenses and telecentric lens systems, in a manner in which substantially more of the light from the light source compared with the systems in the ""624 Patent is directed past the component and collected for measurement, allowing for a sharper component image on the detector. The use of these optical expedients allows the power requirements on a light source to be reduced by a factor of over one-hundred times over the systems in ""624 Patent. With this reduction in required brightness levels, other light sources besides lasers such as light emitting diodes (LEDs) and incandescent bulbs can be effectively used in component alignment. The ""279 Application discloses how to determine the orientation of a component from the detector measurements as a function of rotational angle.
Copending application Ser. No. 08/372,567 (hereinafter the ""567 Application) filed Jan. 13, 1995, assigned to one of the assignees of the present invention, entitled Method and Apparatus for Electronic Component Lead Measurement Using Light Based Sensors on a Component Placement Machine, incorporated herein by reference, adapts the features of the sensors in the ""634 Patent and the ""279 Application in a innovative, sophisticated way to obtain accurate measurements on the positions of leads on the component. For applications where a higher level of quality control is necessary or desired, such a lead measurement system can be used in place of a component (body) alignment system as described in the preceding paragraphs.
The systems described by the ""634 Patent, the ""279 Application and the ""567 Application are well suited to the high accuracy placement of components onto circuit boards. Furthermore, these entire sensing systems can be integrated into the placement head to provide for more rapid component placement. The angular orientation of the component can be achieved with an accuracy of better than 0.03 degrees and lateral positioning of the component and/or the leads can be achieved to an accuracy of better than 0.001 inches. Yet, these systems were not efficiently integrated into the placement head to provide the optimal overall use of the available resources.
A component sensing system that reduced measurement times without significant modifications of existing hardware would find wide application. Moreover, a component sensing system that could commence movement of a component toward the circuit board for placement once the component has been raised to a safe position, yet before the component is raised into position for measurement, would increase system efficiency. Such a system would also optimally use the measurements made to orient the component to determine if the proper component has been correctly picked up by the system. Furthermore, an optimal system would be self checking to alert the operators when maintenance was required.
The present invention is concerned with improved methods for the placement of circuit board components using component placement machines. These improved methods are advantageously used with focused light based sensor systems. When the sensor is attached to the component placement head, the quill of the component placement head optimally includes a projection or notch at a selected height. As the component is being raised on the end of the quill, the discontinuity on the quill is detected as it passes through the sensor. The height of the discontinuity is selected so that its detection by the focused light based sensor indicate that the component has reached a safe height such that the component placement head can begin motion toward the location for the placement of the component without risk of damage to either the quill or the component.
A second improved method is appropriate for sensors which align components based on the shadows cast by the body of the component. Improved efficiency will result whether these focused light base sensors are mounted on the component placement head or not, although the preferred embodiments will have the sensors attached to the component placement head. The alignment times are reduced without the sacrifice of accuracy by using relatively constant high angular velocity for rotating the component when it is in the light path of the light base sensor. The use of a relatively constant velocity allows for the correction for the time lag in obtaining the orientation of the component. The ability to make this correction allows for the use of relatively high angular velocities without resulting in significant errors.
The alignment of the component based on measurements of either the body or the leads of the component using a sensor employing focused light can provide information related to the dimensions of the component. This information on the dimensions of the component can be compared against known dimensions of the type of component expected to be on the placement head. If the measured dimensions of the component are not within specified tolerances of the known dimensions of the expected components, the component placement head can reject the particular component and thereby avoid a placement error. Therefore, placement errors can be reduced using information already available from the alignment of the component.
The focused light based sensor system is self-inspecting to reduce placement errors and downtime while minimizing the necessary intervention from an operator. Self-inspection is performed by taking measurements with the sensor while no component is in the light path. These unobstructed measurements provide an indication of the performance of the system without the complication of the light beam being blocked partially by the component. These measurements may or may not be made with the quill in the light path as long as the measurements are performed consistently from time to time. Once one of these unobstructed measurements is obtained, the results can be compared against absolute values for the measurements and/or similar measurements made at an earlier time. This comparison will indicate whether the system is performing adequately or whether maintenance is required. The system can be designed to further suggest the type of maintenance required, for example, cleaning, repair, or replacement.